Relationship between the band gaps and formation energies of 3d and 4d transition metal-doped TiO2. The elements colored in black are elements that do not form the impurity levels in the band gap. The elements colored in red are elements that form the impurity levels in the band gap but do not form the middle level. The elements colored in blue are elements that occur in the impurity levels in the band gap and form the middle levels. The horizontal dashed line indicates 0 eV, and the vertical dashed line represents the calculated band gap of pure TiO2 (2.21 eV).
Wang et al. Nanoscale Research Letters 2014 9:46 doi:10.1186/1556-276X-9-46